In general, exhaust gas is discharged from an engine through an exhaust manifold. Before the exhaust gas is discharged into atmosphere through a tail pipe, it is purified through a catalytic converter installed in an exhaust pipe and then passes through a muffler to attenuate noise. The catalytic converter removes pollutants contained in the exhaust gas. Further, a particulate filter is mounted in the exhaust pipe to capture particulate matters (PM) contained in the exhaust gas.
A denitrification catalyst (DeNOx Catalyst) is a type of catalytic converter that removes nitrogen oxide (NOx) contained in the exhaust gas. When a reducing agent, such as urea, ammonia, carbon monoxide, and hydrocarbon (HC), is in the exhaust gas, the nitrogen oxide contained in the exhaust gas is reduced in the denitrification catalyst through an oxidation-reduction reaction with the reducing agent.
Recently, a lean NOx trap (LNT) is used as the denitrification catalyst. When the LNT operates under a lean environment in which an air/fuel ratio for the engine is low, the LNT adsorbs the nitrogen oxide contained in the exhaust gas. When the LNT operates under a rich environment in which the air/fuel ratio for the engine is high, the LNT desorbs the adsorbed nitrogen oxide and reduces the desorbed nitrogen oxide and the nitrogen oxide contained in the exhaust gas.
However, the LNT cannot remove the nitrogen oxide contained in the exhaust gas when a temperature of the exhaust gas is high (e.g., a temperature of the exhaust gas is higher than 400° C.). In particular, when the particulate filter for capturing particulate matters (PM) contained in the exhaust gas is regenerated or sulfur deactivated by the LNT is removed, the temperature of the exhaust gas becomes very high. Therefore, the nitrogen oxide contained in the exhaust gas may be discharged to the outside of the vehicle.
Recently, as the emission regulations become stricter, a separate DeNOx catalyst (e.g., composite catalyst unit (diesel particle filter with selective catalytic reduction coating (SDPF)) is used together with the LNT.
In the related art, the SDPF is installed at a rear end of the LNT, and as a result, a predetermined distance between the LNT and the SDPF needs to be maintained in order to uniformly input urea to the SCR.
For this reason, a thermal loss occurs and a NOx removal rate of the SCR deteriorates while the exhaust gas discharged from the LNT reaches the SDPF. A method of increasing a temperature of the exhaust gas by using post-injection is sometimes used to solve the above problem, but in this case, fuel efficiency of the vehicle decreases.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.